Later we learn that mass is determined by energy and momentum, both of which are conserved, so mass must be conserved too. Not in a meaningful sense - the squa

Later we learn that mass is determined by energy and momentum, both of which are conserved, so mass must be conserved too. Not in a meaningful sense - the square of the total momentum 4-vector is of course conserved (and even frame-invariant), but it's not equal to the sum of the masses of the constituents: M² = (E₁ + E₂)² - (p₁ + p₂)² = E₁² - p₁² + E₂² - p₂² + 2E₁E₂ - 2p₁p₂ = m₁² + m₂² + 2(E₁E₂ - p₁p₂)